decoding GADTs gives internal error: stg_ap_v_ret

May be related to #4987 (closed), but this time I have a reproducible case.

So far I found out that the %INFO_TYPE(%STD_INFO(info)) in stg_ap_v is 4 (==CONSTR_2_0) just before barfing.

The unsafe cast is just to generalize to the return type, but seem to be crucial. When I make TypedPair unparametrized or remove the TPInt constructor (but remaining a GADT), the crash goes away.

The problem does not happen with GHCi (with this testcase), but I have seen the same crash in GHCi too with a more involved code example.

I compiled vanilla: ghc internal-error.lhs; ./internal-error

I did not test with older compilers.

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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